1. Field of the Invention
The present invention relates to the field of detection apparatuses used to screen for the presence of explosives and other chemical entities.
2. Background Information
Mass spectrometry (MS) is one of the most important analytical methods for analyzing samples and materials for chemical composition. A main component of a MS instrument is the ionization source. Ionization sources are also used for other analytical instrumentation such as ion mobility spectrometry (IMS). There are many types of ionization sources used in these instruments depending on the analysis applications, and also the methods and types of molecules that are being analyzed.
There has been new developments in ionization sources for MS instruments that are based on atmospheric pressure ionization (API). These type of sources are typically employed for MS analyses that use liquid chromatography (LC) for sample preparation and separation. The most common ionizers are electrospray ionization (ESI), atmospheric pressure chemical ionization (APCI) and atmospheric pressure photoionization (APPI). Recent developments have included developing multimode sources including some combination of these sources.
Whereas significant development of ionization sources has been made in recent years for LC-MS, very little new development has been made for the related analytical instrumentation of gas-chromatography (GC) MS (GC-MS). The ionizers for GC-MS typically occur at less than atmospheric pressure. The method of chemical ionization (CI) is based on introducing some external source of gaseous molecules (e.g., methane, methanol, ethanol, etc.) into a chamber in which a discharge is formed. These ions then transfer their charge to molecules that might be eluting from a GC column. The sample eluting from the GC column is vaporized due to the heat of the GC column and is therefore in gaseous form when entering the ionization volume, which for CI is at about 1 torr to optimize the discharge and charge transfer process.
The more common method of ionization for GC-MS is electron ionization (EI), which is based on creating a stream of electrons that are accelerated to some energy by electrical fields and crossing it with a sample that elutes from a GC column. The typical source of the electrons is a heated filament that when heated by passing a current through the wire causes thermionic emission of electrons. This process is essentially the same as what occurs in an incandescent light bulb. In order for the filament to have reasonable lifetime against burning out due to gaseous bombardment and oxidation, it must be in a vacuum region, typically less than 10−5 torr. The filaments can be coated with various materials (e.g., thorium) that increase the efficiency of thermionic emission of electrons allowing the filament to be operated at somewhat reduced temperatures, however, the tolerance to reduced vacuum conditions (i.e., higher pressures) is not significant. There are various types of devices that are referred to as cold cathodes that can improve on the tolerance of electron emitters to reduced vacuum conditions; however, again to achieve reasonable lifetimes, the pressures are on the order of 10−4 torr or less.
Because of the limitation of standard EI sources with pressure there have been some ion source designs that attempt to generate electrons at high vacuum and then focus them through slits into a tube or other chamber where the sample is maintained at higher pressure. These closed ion sources achieve a higher density of electrons and gaseous sample; however, the intensity of the electron beam is greatly reduced due to inefficiencies in the transmission through the slit.
The process of photoemission of electrons off of surfaces is generally known in the art and the explanation of the process was first revealed by Einstein in his Nobel Prize winning work on the photoelectron effect. Einstein showed that materials such as metals have work functions for electron detachment and that the energy of the electron E detaching from a material of work function W due to the impingement of a photon of energy hv is given by the simple equationE=hν−W The condition for emitting an electron is therefore hν≧W. The typical value of W for metals and other photocathode materials is in the 4-5 eV range. The upper range corresponds to photons of wavelength about 250 nm.
The physical process of photoemission of electrons is used routinely in devices such as photodiodes and photomultiplier tubes. These devices measure the intensity of light by converting photons that hit the photocathode surface into electrons that can be measured by current measuring devices.
The development of photoemission as an ionization source; however, is not common. A few publications discuss methods based on directing a UV laser at a metal surface in vacuum to generate electrons that are then accelerated through a molecular beam to achieve ionization (Rohwer et al. 1988; Syage et al. 1989; Boyle et al. 1991; Boesl et al. 1994). However lasers are very complex and expensive and therefore not practical for a general purpose chemical analysis device. Furthermore the UV lasers are invariably pulsed lasers and therefore do not allow a continuous beam of electrons, which is desirable for most common methods of GC-MS analysis and direct sampling MS analysis.
There are very few developments of photoemission of electrons as an ionization source for MS, IMS, or any analytical method. U.S. Pat. No. 4,574,004 issued to Schmidt-Ott et al discloses a method for charging particles by irradiating with light that causes photoemission of electrons from the particle, but this does not use the resulting electrons as an ionization source. U.S. Pat. No. 5,461,280 issued to Kane discloses a method to enhance a cold-cathode emitter by irradiating the surface with photons thereby allowing electron emission with a lower applied potential electric field. However, this method does not address total electron flux, or use as an ionization source.
U.S. Pat. No. 4,713,548 issued to Kim discloses a photoelectron emitter comprising a UV light source and photocathode to create negative ions; however, this invention does not disclose or teach how to achieve positive ionization or a combination of ionizations. U.S. Pat. No. 7,196,325 issued to Syage discloses a photon source and photocathode for ionization in combination with a glow discharge source. However, this patent, nor any of the others referenced disclose or teach how to achieve a narrow energy electron beam, convenient replacement without disrupting the vacuum, integration with a GC sampling system, nor the combination of low and medium pressure operation along with the combination of PE-EI and PI.